This invention relates generally to apparatus and methods for controlling the frequency of light output from an optical signal source. This invention is particularly related to apparatus and methods for controlling the frequency of optical signals output from coherent light sources. Still more particularly this invention relates to apparatus and methods for controlling the frequency output from an optical signal source used in a fiber optic rotation sensing system to stabilize the scale factor that relates the Sagnac phase shift to the rotation rate.
A fiber optic ring interferometer typically comprises a loop of fiber optic material having counter-propagating light waves therein. After traversing the loop, the counter-propagating waves are combined so that they constructively or destructively interfere to form an optical output signal. The intensity of the optical output signal varies as a function of the type and amount of interference, which is dependent upon the relative phase of the counter-propagating waves.
Fiber optic ring interferometers are particularly useful for sensing rotations. Rotation of the loop creates relative phase difference between the counter-propagating waves, in accordance with the Sagnac effect, with the amount of phase difference being a function of the angular velocity of the loop. The optical output signal produced by the interference of the counter-propagating waves varies in intensity as a function of the rotation rate of the loop. Rotation sensing is accomplished by detecting the optical output signal and processing the optical output signal to determine the rotation rate.
In order to be suitable for inertial navigation applications, a rotation sensor must have a very wide dynamic range. The rotation sensor must be capable of detecting rotation rates as low as 0.01 degrees per hour and as high as 1,000 degrees per second. The ratio of the upper limit lower limits to be measured is approximately 10.sup.9.
The development and practical implementation of rotation sensing systems using optical signals require stability in the optical pulses input to the optical fibers. Optical sensing systems may use semiconductor diode lasers or superluminescent diodes as light sources. Broadband semiconductor light sources have been used in fiber optic rotation sensors to reduce noise arising from backscattering in the fiber and for reducing errors caused by the optical Kerr effect. For a high precision fiber optic rotation sensor the wavelength of the light source must be stabilized since the scale factor of the sensor depends upon the source wavelength. A navigation grade rotation sensor requires wavelength stability of about one part in 10.sup.6.
All solid state coherent light sources include two polished parallel faces that are perpendicular to the plane of a junction of the p-type and n-type semiconductors. The combination of electrons injected from the n-region into the p-region with holes, or positive charge carriers, in the p-region causes the emission of coherent light. The emitted light reflects back and forth across the region between the polished surfaces and is consequently amplified on each pass through the junction.
The wavelength of the light emitted from a solid state coherent light source varies as functions of the operating temperature and the injection current applied. Effective use of coherent light sources in an optical rotation sensor requires an output of known wavelength.
Superluminescent diodes used as light sources in fiber optic rotation sensors typically have fractional linewidths of about 10,000 ppm. They also have operating lifetimes of about 100 hours and provide about 500 .mu.W or less optical power into an optical fiber. SLD's have linewidth to frequency stability ratios of about 10,000 and require relatively high injection currents that typically exceed 100 mA. The short operating lifetime and excessive linewidths makes SLD's unacceptable for fiber optic rotation sensors, which should operate reliably for thousands of hours without source replacement.
Some familiarity with polarization of light and propagation of light within a guiding structure will facilitate an understanding of the present invention. It is well-known that a light wave may be represented by a time-varying electromagnetic field comprising orthogonal electric and magnetic field vectors having a frequency equal to the frequency of the light wave. An electromagnetic wave propagating through a guiding structure can be described by a set of normal modes. The normal modes are the permissible distributions of the electric and magnetic fields within the guiding structure, for example, a fiber optic waveguide. The field distributions are directly related to the distribution of energy within the structure. The normal modes are generally represented by mathematical functions that describe the field components in the wave in terms of the frequency and spatial distribution in the guiding structure. The specific functions that describe the normal modes of a waveguide depend upon the geometry of the waveguide. For an optical fiber, where the guided wave is confined to a structure having a circular cross section of fixed dimensions, only fields having certain frequencies and spatial distributions will propagate without severe attenuation. The waves having field components that propagate with low attenuation are called normal modes. A single mode fiber will propagate only one spatial distribution of energy, that is, one normal mode, for a signal of a given frequency.
In describing the normal modes, it is convenient to refer to the direction of the electric and magnetic fields relative to the direction of propagation of the wave. The direction of the electric field vector in an electromagnetic wave is the polarization of the wave. If only the electric field vector is perpendicular to the direction of propagation, which is usually called the optic axis, then the wave is a transverse electric (TE) mode. If only the magnetic field vector is perpendicular to to the optic axis, the wave is a transverse magnetic (TM) mode. If both the electric and magnetic field vectors are perpendicular to the optic axis, then the wave is a transverse electromagnetic (TEM) mode.
None of the normal modes require a definite direction of the field components. In a TE mode, for example, the electric field may be in any direction that is perpendicular to the optic axis. In general, a wave will have random polarization in which there is a uniform distribution of electric field vectors pointing in all directions permissible for a given mode. If all the electric field vectors in a wave point in only a particular direction, the wave is linearly polarized. If the electric field consists of two orthogonal electric field components of equal magnitude and a phase difference of 90.degree., the electric field is circularly polarized, because the net electric field is a vector that rotates around the propagation direction at an angular velocity equal to the frequency of the wave. If the two linear polarizations are unequal or have a phase difference other than 90.degree., the wave has elliptical polarization. In general, any arbitrary polarization can be represented by the sum of two orthogonal linear polarizations, two oppositely directed circular polarizations or two counter rotating elliptical polarizations that have orthogonal major axes.
An optical fiber comprises a central core and a surrounding cladding. The refractive index of the cladding is greater than that of the core. The diameter of the core is so small that light guided by the core impinges upon the core-cladding interface at angles less than the critical angle for total internal reflection.
The boundary between the core and cladding is a dielectric interface at which certain well-known boundary conditions on the field components must be satisfied. For example, the component of the electric field parallel to the interface must be continuous. A single mode optical fiber propagates electromagnetic energy having an electric field component perpendicular to the core-cladding interface. Since the fiber core has an index of refraction greater than that of the cladding and light impinges upon the interface at angles greater than or equal to the critical angle, essentially all of the electric field remains in the core by internal reflection at the interface. To satisfy both the continuity and internal reflection requirements, the radial electric field component in the cladding must be a rapidly decaying exponential function. An exponentially decaying electric field is usually called the "evanescent field."
The velocity of an optical signal depends upon the index of refraction of the medium through which the light propagates. Certain materials have different refraction indices for different polarizations. A material that has two refractive indices is said to be birefringent. The polarization of the signal propagating along a single mode optical fiber is sometimes referred to as a mode. A standard single mode optical fiber may be regarded as a two mode fiber because it will propagate two waves of the same frequency and spatial distribution that have two different polarizations. Two different polarization components of the same normal mode can propagate through a birefringent material unchanged except for a velocity difference between the two polarizations.
There are a number of birefringent materials. For example, depending on their structure and orientation to the light propagating through it, certain crystals are circularly birefringent; and other crystals are linearly birefringent. Other types of crystals, such as quartz, can have both circular birefringence and linear birefringence.
Stabilization of the scale factor is critical to the performance of a high accuracy fiber optic gyroscope. The scale factor, which relates the angular rotation rate of the sensor to the Sagnac phase shift, is sensitive to changes in the length of the fiber and to variations on the operation wavelength of the source. In superluminescent diodes (SLDs), variations in the emission wavelength are caused by thermal fluctuations in the active region caused by changes in the ambient temperature and in the interaction current. Typically, the temperature dependence of the SLD emission wavelength is about 0.2 namometers per .degree. C. To obtain the required wavelength stability by thermal means alone would require the temperature of the SLD to be held constant to a few millidegrees. The need for such stringent temperature stability can be obviated by using a reference interferometer to continuously monitor the emission spectrum of the SLD. In this scheme, changes in the emission spectrum generate an error signal which is processed and fed back to the diode to hold the wavelength constant.
Lasers, with their long coherence length, are readily stabilized by using a temperature stabilized scanning Fabry-Perot interferometer to lock the cavity length. However, the large spectral bandwidth of the SLD precludes the use of optical spectrum analyzers to monitor the emission wavelength of the diode. The mirrors of a Fabry-Perot interferometer to analyze to typical 10 nm emission bandwidth of an SLD would have to be spaced approximately 1 .mu.m apart, which is impractical.
Previously proposed methods for stabilizing the SLD wavelength use either an in-line technique in which the dispersion of the fiber is used to monitor the SLD wavelength or a break out method in which the SLD emission is monitored independently. In the first method, a Bragg cell is required to generate the large frequency shifts needed to confidently utilize fiber dispersion. Such bulk optic components placed in line with the sensor would introduce alignment problems which could become more pronounced with temperature cycling.
A practical wavelength stabilization scheme must take into account the volume budget of the gyro and should be capable of being packaged within the gyro housing. This constraint limits the volume of the wavelength stabilization device to a maximum of a few cubic centimeters.